(1) Field of the Invention
The present invention generally relates to a storage device, and more particularly to a storage device in which a source power is supplied from a power supply of a host system and an access is sent from the host system to read data from or write data onto a recording medium within the storage device.
(2) Description of the Prior Art
Storage devices such as magnetic disk drives have been connected to host computers for a purpose of storing information. Recently, a computer system in which a storage device is connected as the external storage having a smaller size and a lighter weight has been developed and put into practical use. In addition, a small-size computer system including such a small-size storage device is powered by a battery, instead of by being plugged into an electric outlet.
Generally, the magnetic disk drives are provided with a read/write mechanism which includes a SPM (spindle motor) used to rotate a magnetic disk and a VCM (voice coil motor) used to move a magnetic head. This read/write mechanism is actuated by using a source power supplied from the battery-powered host computer, so that a read/write operation is carried out to read data from or write data onto a magnetic disk within the magnetic disk drive.
The battery-powered host computer mentioned above supplies the source power to the external devices, including the storage device, which are connected to the host computer.
In the above storage device which is connected to the battery-powered host computer, when the source power supplied from the host computer is varied, an electric current flowing through the read/write mechanism is also varied. If the remaining quantity of electric power contained in the battery is small, the source voltage supplied to the storage device becomes low and the electric current through the read/write mechanism becomes too small to carry out a read/write control procedure.
When the source power supplied from the battery to the read/write mechanism is low, the storage device will lack sufficient electric energy to start or continue the rotation of the VCM or the SPM of the read/write mechanism. When the storage device lacks a sufficient supplied source power, it is impossible to carry out a seek control procedure of the VCM on the read/write mechanism, for example, in accordance with seek control data.
Japanese Laid-Open Patent Application Nos.5-137393 and 5-258488 disclose a VCM speed control of a storage device, proposed as an optimum VCM speed control, wherein one of predetermined source voltages (e.g., 3 V and 5 V), supplied to the storage device, is selected by detecting a change in the source voltage from the host computer. In the storage device disclosed in the above publications, a VCM speed control procedure of the read/write mechanism is carried out in accordance with control data of a VCM speed control table related to the selected source voltage.
Generally, a rated quantity of source power is supplied to the magnetic disk drive. However, the host computer may not recognize a consumption power actually used by the magnetic disk drive. When the remaining quantity of electric energy contained in the battery of the host computer is small, a warning related to the lack of electric energy in the battery is displayed on the host computer. When the remaining quantity of the electric energy contained in the battery is smaller than a lower electricity limit that allows the host computer to normally run, the operations of the host computer and the storage device are stopped.
In the case of the conventional storage device mentioned above, one of the predetermined source voltages, supplied to the storage device, is selected by detecting a change in the source voltage from the host computer, and a switching of one of the source voltages to another source voltage is abruptly performed. There is a problem that the contents of a register of a control unit of the host computer or data stored in a memory of the host computer might be changed at random or completely lost due to the abrupt switching of the source voltage. In addition, noises might be produced due to the abrupt switching to cause a read/write error or another malfunction of the storage device.
In the case of the above conventional storage device, when the remaining quantity of electric power contained in the battery of the host computer becomes small, the storage device might be unable to continue to run a normal operation. The conventional storage device under such a condition has a problem that data which is to be written onto the recording medium abnormally resides on a memory of the host computer due to the lack of electric power contained in the battery. In addition, there is a problem that the conventional storage device is not capable of performing a reduced consumption power mode when the remaining electric power of the battery is small.